Biopotentials are usually recorded using electrodes attached to the body, such as wet (gel) electrodes, or dry electrodes. The electrodes are used to measure biopotentials, which typically have a magnitude in the range of about 1 μV to 10 mV.
Active electrodes have been employed in biopotential acquisition systems, in which the electrodes are integrated with amplifiers for the suppression of interference picked up from cables. The active electrode based system is robust to cable motion artifacts and interferences, which makes it suitable for dry electrode applications.
In one example, an active electrode includes a passive electrode and a pre-amplifier that are integrated within the same package or board, which can be placed very close to the skin to extract the low-level biopotential signals. In this way, the signal path length between the electrode and the pre-amplifier may be minimized, maintaining the highest possible input impedance of the amplifier and lowest possible noise pick-up from electromagnetic fields. Furthermore, the output of the active electrode forms a low-impedance node and the interference and motion artifacts obtained by cable movement and electromagnetic fields in the vicinity can both be reduced when compared to a conventional passive electrode interface.
FIG. 1 shows the basic active electrode system. A reference active electrode 10 comprises an amplifier 12 and an analogue to digital converter 14. A first signal active electrode 16 comprises an amplifier 18 and an analogue to digital converter 20. The gain of the reference active electrode amplifier is shown as A and the gain of the signal active electrode amplifier is shown as A+ΔA. Thus, there may be a gain mismatch between the active electrodes.
Common mode interference is one of the problems in such active electrode biopotential signal acquisition systems. For example, biopotential signals can be affected by interference currents derived from the mains power supply lines, known as “common mode aggressors”. The mains frequency generally falls within the frequency range of interest of biomedical signals, which makes such common mode signals a particular problem. For example, an ECG signal has its main frequency components in the range 0.5 Hz to 40 Hz, but signal information up to around 100 Hz or even 200 Hz is desired.
The common mode interference is represented by the signal source 22 in FIG. 1.
The interference signals can have larger amplitudes than the biopotential signals that are to be measured. This causes the biopotential amplifier to have very high common mode rejection ratio (CMRR) in order to eliminate the common mode interference signals appearing at the output of the amplifier, and thereby reject the 50 Hz or 60 Hz common mode interferences whilst extracting the biopotential signals.
One potential problem is that the common mode interference at the inputs of the active electrodes can be converted to a differential mode error at the outputs of active electrode pairs due to the voltage gain mismatch between the active electrode pairs, as well as the contact-impedance mismatch. The output error can have significant amplitude when compared to the amplitude of the biopotential signals. The CMRR of active electrode systems is usually limited by this voltage gain mismatch (VCM *ΔA) between the signal active electrode and the reference active electrode, which in return is due to the process variation and component mismatch.
The article “Enhancing Interference Rejection of Preamplified Electrodes by Automated Gain Adaptation” of Thomas Degen, in IEEE Transactions on Biomedical Engineering, Vol. 51, o. 11, November 2004, discloses adaptation of the gain of an analogue differential amplifier which follows the active electrodes in response to a detected amplified common mode signal. A common mode feedback system is also disclosed by which a common mode signal is fed back to the patient (for example via a driven right leg electrode).